The present invention relates to a hydraulic continuously variable speed transmission comprising a constant displacement hydraulic pump and a variable displacement hydraulic motor, and more particularly to a continuously variable speed transmission provided with a direct clutch for blocking a hydraulic closed circuit, through which the pump is hydraulically connected to the motor, to render the both in a direct connecting state when a speed reduction ratio of the variable speed transmission has become 1.
Continuously variable speed transmissions have conventionally been proposed in publications in the Japanese Official Gazette such as Japanese Patent Publication No. 32(1957)-7159, Japanese Patent Publication No. 56(1981)-50142 and the like, wherein a constant displacement hydraulic pump is connected to an input shaft, oil delivered from the pump is fed to a variable displacement hydraulic motor through a closed circuit, and the hydraulic pump is driven to drive an output shaft connected thereto.
It is known to provide a direct clutch which is capable of blocking the above hydraulic closed circuit, whereby the direct clutch device blocks the hydraulic closed circuit to integrally rotate the pump and the motor when the swash plate angle for variably controlling the displacement of the hydraulic motor is minimized (and the speed reduction ratio of the variable speed transmission becomes 1).
A method for controlling a continuously variable speed transmission having a direct clutch device, as disclosed in Japanese Patent laid-open Publication No. 54(1979)-134252 and Japanese Patent laid-open Publication No. 55(1980)-14312, comprises controlling the speed reduction ratio in such a way that the engine speed coincides with a reference engine speed corresponding to the throttle opening to obtain minimum fuel consumption and blocking the closed circuit by the direct clutch device to integrally rotate the pump and the motor when the speed reduction ratio becomes minimum or 1.
In controlling the direct clutch, as described above, when the direct clutch blocks or opens the closed circuit (when it is switched ON or OFF) at a speed reduction ratio of 1, the load of the engine is changed because of change of the hydraulic thrust forces acting on the motor plungers and change of the volumetric efficiency. Therefore the following problems may be produced in the ON/OFF control of the direct clutch.
After the direct clutch is switched ON at a speed reduction ratio of "1", a direct clutch disconnecting reference engine speed is set according to the engine throttle opening and then the direct clutch is switched OFF when the actual engine speed becomes smaller than the reference engine speed. When the direct clutch is switched OFF, the hydraulic thrust forces onto the motor plungers are, as described above, restored (increased) and the volumetric efficiency is decreased. Accordingly the engine speed is increased and the actual engine speed becomes higher than the reference engine speed to switch the direct clutch ON causing a "hunting" phenomenon. When tires are locked by aggressive braking in the state of the direct clutch being switched ON (the closed circuit being blocked), the engine may be stalled since it takes at least some time to relieve the blocking of the closed circuit by the direct clutch valve (i.e., it takes some time to switch the direct clutch valve OFF).
The reason for engine stall during aggressive braking in a prior art transmission equipped vehicle is that the vehicle speed is reduced with the direct clutch valve being switched ON and with the speed reduction ratio being minimum. Because the accelerator pedal is released during aggressive braking to make the throttle opening nearly fully closed, therefore the reference engine speed is lowered according to the throttle opening. In addition, since the outlet of the hydraulic pump is closed as long as the direct clutch valve is switched ON, the main clutch valve, which is used to communicate the outlet of the hydraulic pump with the inlet thereof, is inoperative even if it is opened. Therefore, the problem in the prior art of the engine stall during aggressive braking cannot be solved.